1. Field of the Invention
This invention relates to a step-up type DC-DC converter and a method for controlling the step-up type DC-DC converter. More specifically, this invention relates to a DC-DC converter that is capable of being started up by a low input voltage and that has high efficiency.
2. Description of Related Art
A portable electronic device that uses batteries as a power source includes a DC-DC converter by which the voltage of the batteries is raised to a voltage necessary for the electronic device. Especially in a small-size portable electronic device, the number of batteries that can be mounted on the electronic device is small, and hence an extremely low voltage must be raised to a desired voltage of the electronic device. Additionally, since a battery-operated electronic device is required to be operated by a small number of batteries for a longer time, a DC-DC converter of the electronic device is required to have higher efficiency. In order to actuate the DC-DC converter from the state of a low input voltage and allow the DC-DC converter to work with high efficiency, Japanese unexamined patent publication No. H08(1996)-186980 proposes the following method.
According to circuits disclosed by the publication '980 shown in FIG. 4, an oscillation/starting circuit 109 causes switching of a bipolar transistor 105b to generate a power of boosted voltage that is fed to a load when the voltage of a power source that is fed to the load is lower than a voltage value by which a MOS transistor can be turned on/off. When the boosted voltage reaches a proper voltage by which an oscillation/driving circuit 108 of a DC/DC converter of the MOS transistor is operated, the switching operation of the bipolar transistor 105b is stopped, and the MOS transistor 105a is actuated.
Japanese unexamined patent publication Nos. H03(1991)-74169, H01(1989)-295665, and 2001-251849 are disclosed as techniques related to the above publication '980.